Wednesday, August 19, 2015

A Proper IF Rig.....Finally!

It has taken me way too long to get to this point. I have been working off and on (mostly off, I'm afraid) working with and building my own transverters, for almost six years now. One obstacle I've encountered again and again is the need for a proper IF rig, with an attenuator that switches in and out of the RF path during transmit automatically.

For those unfamiliar with how a transverter works, basically you start with a local oscillator (LO) at frequency X. You then mix that with an intermediate frequency (IF) Y to get the desired operating frequency X + Y (or less commonly X - Y). So for example if you start with an LO running at 756 MHz and mix that with the output from an IF rig at 146 MHz, presto! you're operating at 902 MHz.

I have found out the hard way that transverters are fiddly things. The mixer that combines that IF and LO, for example, can only take a few milliwatts of power. More than once I've blown the mixer on my transverter because I thought I could hook up an IF rig with no attenuator, just to test the receive path. I won't accidentally transmit this time.....really, I won't (zork! smoke......$#%@&!)

I've had a Downeast Microwave (DEM) Transverter Control (TC) kit for a long time now. This board is particularly good for use with the W1GHZ transverters that I'm building, because it can be configured for transverters with a single mixer. Well I finally got the thing built, tested, adjusted, and integrated with the rig. It works great. Why did this take me so long? This is not exactly rocket science, eh? I don't know why. But I'm glad to have it done.

As you can see from the pictures, I have a Yaesu FT-817 and TC board (in a scrounged enclosure)
velcroed together and connected with some stubby cables. A single Power
Pole connector powers both rig and TC with one connection. Included in the enclosure is a SP4T RF relay that I got from K2TER (thanks Bill!), so that with the twist of a rotary switch (not yet wired in) I'll be able to switch my IF to up to four transverters. I also
rigged up a simple pushbutton allowing me to key the FT-817 while I test
the transverters.

So that's the interface. What about the rig itself? I'm using a Yaesu FT-817 because I think it makes just about the perfect IF rig. It can do 28, 144, or 432 MHz IF. It can be modified to transmit out-of-band for odd IF frequencies (legal to do if you're only generating that RF for a transverter intermediate frequency.) And of course it can do HF, VHF and UHF FM, satellite, and a bunch of other fun stuff. I managed to find a reasonably priced FT-817 non-D radio, which was just what I wanted. The non-D has had some problems with blown finals at full power, but I don't care about that because I'll only be transmitting at around 1 watt all the time.

I'm going to modify the FT-817 further to incorporate the N1JEZ/W1GHZ panadapter (I'll write up that project as it happens), which I'll then use with an inexpensive SDR-RTL (more on that later too) to get a waterfall display.

I adjusted the DEM TC board to give me just about 0 dBm (1 mW) out for 1 watt in (that's the L2 power setting on the FT-817.) That lets me drop down to around -3 dBm with the 0.5 watt setting or bump up to 3 or 5 dBm if I need a bit more oomph.

So finally I have an IF rig tightly interfaced to a transverter control. So now I just need to get a W1GHZ transverter on the air.